The drive for robust, high-capacity information networks has resulted in many advances in the field of optical signal routing and processing. While most local networks still rely on electronics, many long distance communications lines are using optical signals to transmit information. Depending on the transmission protocols selected and transmission characteristics of the optical components used, the information-bearing optical signals are contained in a number of channels at predetermined optical frequencies. There are numerous protocols for defining channel parameters, including Wavelength Division Multiplexing (WDM) or Dense Wavelength Division Multiplexing (DWDM) protocols. The waveguides used in these long distance networks are optical fibers, which offer advantages such as low loss, immunity to interference and, most importantly, an extremely large bandwidth.
To transmit information, data is modulated on optical carrier a signals of wavelengths corresponding to the selected channels (e.g., WDM channels). The information-bearing carrier signals are combined at the transmitting end and sent via the optical fiber to the receiving end. Along the way, the signals encounter various active and passive network elements including routing nodes, frequently equipped with repeaters and dispersion compensation elements among others. Traditionally, at many of these nodes the signals are converted back into electronic form for processing. Afterwards, they are converted back into optical signals as they leave the node. Speed, bandwidth and power requirements can be limiting due to this conversion.
The above problems are circumvented in an all-optical network in which the nodes switch optical signals in the different wavelength channels in different directions generally without converting the optical signals into electronic form. Several concepts for all-optical WDM networks have been developed for this purpose. The fundamentals of all-optical routing operations require the ability to discriminate between two signals of wavelengths λ1 and λ2 and to switch them to different optical paths based on their wavelengths. Switches which can perform such operations are known in the art and include, among other, acousto-optically or electro-optically tunable filters and micro-electromechanical systems (MEMS). In addition, all-optical switches should also be able to perform wavelength conversion functions, i.e., switch the two optical signals between different optical carrier wavelengths, either within the immediate network or when transferring to a neighboring network. Such wavelength switches can be used to build wavelength interchangers or wavelength interchanging cross-connects. More information about such switches is provided by S. J. B. Yoo in “Wavelength Conversion Technologies for WDM Network Applications”, Journal of Lightwave Technology, Vol. 14, No. 6, June 1996, pp. 955–66 as well as in U.S. Pat. No. 5,825,517 to Antoniades et al. and in the references cited therein.
In a practical all-optical network the nodes have to be able to perform frequency mixing operations on a large number of optical signals of different wavelengths, i.e., multiple signals contained in different channels. One way to achieve frequency mixing operations on a number of signals at multiple wavelengths is to employ separate discrete single channel devices. Typically, single channel frequency mixing devices employ an optical material exhibiting a nonlinear susceptibility to perform one or more frequency mixing operations. Among other, such operations can include second harmonic generation (SHG), difference frequency generation (DFG), sum frequency generation (SFG), or parametric amplification. For example, it is sometimes useful to perform SHG followed by DFG, which uses the second harmonic generated by SHG. General information about wavelength conversion in multiple WDM channels is provided by Lacey, J. P. R. et al., in “Four-Channel Polarization-Insensitive Optically Transparent Wavelength Converter”, IEEE Photonics Technology Letters, Vol. 9, No. 10, October 1997, pp. 1355–7.
To achieve efficient frequency conversion many devices use quasi-phase-matching (QPM) to counteract the phase slip between the generating nonlinear polarization and the generated or converted optical field as these propagate through the nonlinear optical material. Thus, there is a phase velocity mismatch between the generating polarization and generated optical signals. QPM employs a grating in the nonlinear material to periodically compensate for this phase velocity mismatch. There are several methods for producing and tuning such QPM gratings and general information on the theory and applications of QPM within optical waveguides can be found in Michael L. Bortz's Doctoral Dissertation entitled “Quasi-Phasematched Optical Frequency Conversion in Lithium Niobate Waveguides”, Stanford University, 1995 as well as M. L. Bortz et al., “Increased Acceptance Bandwidth for Quasiphasematched Second Harmonic Generation in LiNbO3 Waveguides”, Electronics Letters, Vol. 30, Jan. 6, 1994, pp. 34–5.
Several prior art references teach the use of QPM for purposes of phasematching signals with do not bear information. For example, U.S. Pat. No. 5,644,584 to Nam et al.; U.S. Pat. No. 5,912,910 to Sanders et al.; U.S. Pat. No. 6,021,141 to Nam et al. and Becouarn, L. et al., “Cascaded Second-Harmonic and Sum-Frequency Generation of a CO2 Laser Using a Single Quasi-Phase-Matched GaAs Crystal”, Conference on Lasers and Electro-Optics, IEEE, Vol. 6, pp. 146–7, 1998 teach conversion of output signals from lasers and conversion of optical signals which do not carry information.
Meanwhile, specific application of QPM based wavelength converters dealing with information-bearing signals and including WDM applications are discussed by C. Q. Xu et al., “1.5 μm Band Efficient Broadband Wavelength Conversion by Difference Frequency Generation in a Periodically Domain-Inverted LiNbO3 Channel Waveguide”, Applied Physics Letters, Vol. 63, 27 December 1993, pp. 3559–61; C. Q. Xu et al., “Efficient Broadband Wavelength Converter for WDM Optical Communication Systems”, Conference on Optical Fiber Communication, IEEE, 20–25 February 1994; M. H. Chou et al., “1.5-μm-Band Wavelength Conversion Based on Cascaded Second-Order Nonlinearity in LiNbO3 Waveguides”, IEEE Photonics Technology Letters, Vol. 11, No. 6, June 1999, pp. 653–5; as well as M. H. Chou et al., “1.5-μm-Band Wavelength Conversion Based on Difference-Frequency Generation in LiNbO3 Waveguides with Integrated Coupling Structures”, Optics Letters, Vol. 23, No. 13, Jul. 1, 1998, pp. 1004–6. In addition, U.S. Pat. No. 5,434,700 to Yoo teaches an all-optical wavelength converter which uses an optical waveguide with regions having differing nonlinear optical susceptibilities such that the regions form a quasi-phase-matching grating. This single channel device is proposed for use in optical WDM networks to convert a single signal frequency.
Further, U.S. Pat. No. 5,815,307 to M. Arbore et al., and U.S. Pat. No. 5,867,304 to Galvanauskas et al. teach the use of aperiodic QPM gratings. In particular, these references teach the use of aperiodic QPM gratings in nonlinear materials for simultaneous frequency conversion and compression of optical pulses.
Unfortunately, setting up a number of single channel devices to perform frequency mixing operations on a number of signals in parallel is usually impractical and introduces excessive losses in the network. This is especially true when the number of channels or wavelengths is large, e.g., in the case of DWDM. Hence, it would be a significant advance to provide an apparatus and method for performing frequency mixing operations on signals in many wavelength channels simultaneously without having to use a number of dedicated single channel devices. Specifically, it would be very useful to have such apparatus tuned for frequency mixing operations using more than one short wavelength signals by having corresponding short wavelength channels.